FIG. 1 shows an inspection system 10 having a prior art monochrome line scan video camera 12 that generates an electrical video signal representing objects aligned with a line scan area 14 having an aspect ratio of about 1024:1. Inspection system 10 inspects line segments 16 (shown as cross-hatching) of articles passing through line scan area 14. Inspection system 10 is capable of inspecting articles of various shapes and sizes such as, for example, french fry potato strips, lima beans, peas, carrots, or sliced carrots. For purposes of description, inspection system 10 is shown inspecting generally elongated articles 18, which are representative of french fry potato strips. Line segments 16 correspond to the portions of articles 18 intersected by line scan area 14.
Inspection systems used by the food product industry include a conveyor belt 22 that carries in a direction 24 large numbers (not shown) of generally elongated articles 18, such as french fried potato strips or green beans, distributed over and substantially covering conveyor belt 22. Elongated articles 18 are illuminated by a nominally white light source 26, have lengths generally aligned with direction 24, and are oriented generally perpendicular to a scanning axis 28 of line scan camera 12. Line scan camera 12 generates a video signal corresponding to the source light reflected by line segments 16 of articles 18 and the portion of conveyor belt 22 positioned in line scan area 14 during an image cycle or frame. Video signals representing substantially complete two-dimensional images of articles 18 are generated by scanning the articles during successive image frames while conveyor belt 22 carries the articles past line scan area 14. In one embodiment, inspection system 10 identifies from the video signals defects in articles 18, thereby enabling excision of the defects from the articles or the removal of articles having defects from conveyor belt 22.
Conventional line scan cameras typically employ a single 1024.times.1 photodetector array that provides a single gray-scale (i.e., black-and-white) representation of the scanned articles at an image frame rate of about 1000 Hz. The 1024.times.1 configuration of the photodetector array defines the 1024:1 aspect ratio of line scan area 14 extending across conveyor belt 22 along scanning axis 28.
A disadvantage of such cameras in defect detection systems is that defective and non-defective portions of an article may be indistinguishable in the black-and-white representation. For example, the detection of defects in green beans can be relatively difficult with a black-and-white camera because the colors corresponding to defective and non-defective regions reflect substantially equal amounts of light of different portions of the visible spectrum. Since a black-and-white representation corresponds to overall light intensity, certain defects in green beans are virtually undetectable by a line scan camera employing only one gray-scale.
Certain inspection systems employ conventional color video cameras of the raster scan area array camera type, which uses a two-dimensional (e.g., 512.times.512) array of photodetectors. A raster scan area array camera can work at lower frame rates, but suffers from increased complexity stemming from a need to track the articles through the two-dimensional field of view. As a consequence, such cameras are undesirably complex for high-speed inspection systems of the type used in the food product industry.
Conventional monochrome line scan cameras typically have an objective lens arrangement with a fixed magnification that allows the lens to form an image only of the complete width of the conveyor belt along the scanning axis. As a consequence, particular positions across the width of the conveyor belt are fixedly focused onto specific ones of the photodetectors in the array. Such magnification characteristics provide acceptable performance when the entire width of the conveyor belt is covered by articles to be inspected. Whenever the conveyor belt is not completely covered with articles to be inspected, however, the photodetectors upon which the uncovered portions of the conveyor belt are focused provide no inspection information.
In one type of conventional color line scan camera, each full-color pixel is generated by a triad of separate, closely positioned red-, green-, and blue-component photodetectors. The triads in such a camera are arranged in a line, with the three photodetectors of each triad having either an in-line or a delta configuration. Such cameras typically provide unpredictable edge effects, which are inaccurate representations of the color of the edges of a scanned article. The unpredictable edge effects occur when the edge of the scanned article is focused upon only some or parts of the photodetectors in a triad, thereby causing the triad to generate a false representation of the color or hue of the article. The unpredictable edge effects characteristic of this type of color line scan camera limits the resolution of such cameras in inspection systems.
To make use of all of the photodetectors under such circumstances, certain line scan camera systems may be selectively fitted with different objective lenses having different fixed magnifications. Selecting and fitting objective lenses can, however, be time consuming and require an expensive array of objective lenses to cover the range of possible fields of view.